The present invention relates to an ultrasound diagnostic apparatus and a method of producing an ultrasound image, and in particular, to an ultrasound diagnostic apparatus which performs transmission and reception of ultrasonic waves using a plurality of transducers arranged in an array.
An ultrasound diagnostic apparatus using an ultrasound image has hitherto been put into practical use in the field of medicine. In general, in this type of ultrasound diagnostic apparatus, an ultrasonic beam from a transducer array of an ultrasound probe is transmitted toward a subject, an ultrasonic echo from the subject is received by the transducer array of the ultrasound probe, and the reception signals are electrically processed to produce an ultrasound image.
Normally, after the reception signals obtained by the transducer array of the ultrasound probe are detected, phase matching is performed to produce sample data. An image signal for displaying an ultrasound image is created on the basis of the sample data.
At this time, reception signal detection and phase matching have been performed on the basis of the frequency characteristics of the ultrasound probe in a main lobe maximum sound pressure direction.
On the other hand, since the transducer array of the ultrasound probe has a plurality of transducers arranged in a one-dimensional or two-dimensional array, the angle between one reflection point in the subject and an acoustic radiation surface in each transducer differs between the transducers, and an ultrasonic echo from the reflection point enters a plurality of transducers at different angles. For this reason, the reception signals obtained by the plurality of transducers are different in center frequency, bandwidth, sensitivity, or the like depending on the incidence angle of the ultrasonic echo. Accordingly, phase matching of such reception signals results in degradation in image quality of an ultrasound image.
For example, JP 2005-58321 A discloses an ultrasound diagnostic apparatus in which, when compound scanning is performed to synthesize reception data obtained through transmission/reception of an ultrasonic beam at different transmission/reception angles to obtain an ultrasound image, a center frequency is adjusted in accordance with the transmission/reception angle of an ultrasonic wave, thereby achieving improvement in image quality of the ultrasound image.
However, in the apparatus disclosed in JP 2005-58321 A, the center frequency is changed uniformly in accordance with the angle of the ultrasonic beam in transmission/reception with respect to a plurality of transducers. This is effective for compound scanning in which an ultrasonic beam is transmitted and received at different transmission/reception angles. Meanwhile, there is no effect when an ultrasonic echo from the same reflection point enters the individual transducers at different angles.